The present invention relates to a nickel-metal hydride secondary cell and more particularly, the same having an improved non-sinter type nickel positive electrode. 2. Description of the Related Art
As the up-to-date electronic technology has been advanced ensuring lower energy consumption and improved mounting techniques, most of electronic components which were unquestionably bulky become cordless and portable. Simultaneously, various secondary cells which serve as built-in power sources in electronic components are also requested to hold a higher amount of electrical energy. Among them, an alkali secondary cell has been developed and widely accepted which has a negative electrode fabricated by forming powder of a hydrogen absorbing alloy on an conductive core which serves as a collector. As compared with the cadmium negative electrode in a known alkali secondary cell, such a negative electrode of the hydrogen absorbing alloy provides a higher energy density in weight or volume thus ensuring higher energy storage in a cell. In addition, the characteristics of the cell can be improved and sill be less assaultable on the environment.
On the other hand, a known nickel positive electrode which determines the electricity storage of an alkali secondary cell and is commonly employed in a nickel-cadmium secondary cell, is made by sinter process. More specifically, the sinter-processed nickel positive electrode is made by performing the following sequence of steps. First, carbonyl nickel is coated on a punched metal member. Next, the carbonyl nickel-coated metal member is heated, thus sintering the carbonyl nickel coating, whereby a porous conductive core made is obtained. Then, the core is immersed in an aqueous solution containing nickel ions, whereby nickel hydroxide is formed in the pores open in the surfaces of the core, through chemical and electrochemical reactions. Finally, the core is electrically charged and discharged several times, in an alkali electrolyte. As understood, the disadvantage of this process is that the nickel positive electrode remains unreduced in the production cost due to the elaborate procedures and also, its sinter-processed core becomes increased in the size, preventing the improvement of electrical energy density.
To meet the requirements for higher storage and lower production cost of a secondary cell, a non-sinter type nickel positive electrode has been developed. The non-sinter type nickel positive electrode is fabricated by applying a paste, which is a mixture of nickel hydroxide powder and water with a binding medium, onto a conductive core of different shape formed of a 3-dimensional structure of formed metal, net-shaped metallic fabric, or other material for filling and then, pressing the core into a shape. The non-sinter type nickel positive electrode contains more active materials than the sinter-processed nickel positive electrode, ensuring more storage of electricity. Also, the procedure of production will need no elaborate steps including immersion and chemical procedures, unlike the sinter-process for the sinter-processed electrodes, thus encouraging mass-production and lowering the cost. However, a drawback in the non-sinter type nickel positive electrode is that the distance between each nickel hydroxide powder acting as an active material and a metallic matrix which constitutes the conductive core acting as a collector is about a few to several ten times greater than that of the sinter-processed nickel positive electrode, whereby the collector efficiency will decrease. As the result, the effectiveness of nickel hydroxide will decrease to 50 to 60g of the maximum causing a considerable decrease in the performance as compared with the sinter-processed nickel positive electrode.
To overcome the above drawback, the addition of cobalt compounds is proposed. The cobalt compounds include metallic cobalt, cobalt oxide, and cobalt hydroxide. Particularly, cobalt monoxide (CoO) exhibits a highly active characteristic and the addition of a small amount of the cobalt monoxide will ensure an increase in the effectiveness and collector efficiency of any nickel positive electrode.
The cobalt monoxide is however highly reactive with oxygen in the air and when oxidized to the deep in each cobalt monoxide powder, its inherent property will be eliminated. Also, when the oxidization of the cobalt monoxide is abruptly proceeded, the risk of ignition becomes high. It is then required to fabricate non-sinter type nickel positive electrodes in the inactive atmosphere of e.g. nitrogen or argon gas, which will result in the difficulty of mass-production.
A paste type positive electrode for an alkali storage cell is disclosed in Japanese Patent Disclosure (Kokai) No.61-183868 by Uramoto et al., in which 5 to 30% by weight of cobalt monoxide (CoO) powder is mixed with a composition of active powder materials in the eutectic state consisting of 85 to 95 mol % of nickel hydroxide, 3 to 8 mol % of cobalt hydroxide, and 2 to 7 mol % of cadmium hydroxide. Such a paste type positive electrode which contains highly poisonous cadmium as an active material, will cause disgrace to the environment when disposed without proper arrangement.